This module is a thin wrapper for JSON::XS-compatible modules with a few
additional features. All the backend modules convert a Perl data structure to
a JSON text as of RFC4627 (which we know is obsolete but we still stick to;
see below for an option to support part of RFC7159) and vice versa. This
module uses JSON::XS by default, and when JSON::XS is not available, this
module falls back on JSON::PP, which is in the Perl core since 5.14. If
JSON::PP is not available either, this module then falls back on
JSON::backportPP (which is actually JSON::PP in a different .pm file) bundled
in the same distribution as this module. You can also explicitly specify to
use Cpanel::JSON::XS, a fork of JSON::XS by Reini Urban.

All these backend modules have slight incompatibilities between them, including
extra features that other modules don't support, but as long as you use only
common features (most important ones are described below), migration from
backend to backend should be reasonably easy. For details, see each backend
module you use.

This module respects an environmental variable called
"PERL_JSON_BACKEND" when it decides a backend module to use. If this
environmental variable is not set, it tries to load JSON::XS, and if JSON::XS
is not available, it falls back on JSON::PP, and then JSON::backportPP if
JSON::PP is not available either.

If you always don't want it to fall back on pure perl modules, set the variable
like this ("export" may be "setenv", "set" and
the likes, depending on your environment):

> export PERL_JSON_BACKEND=JSON::XS

If you prefer Cpanel::JSON::XS to JSON::XS, then:

> export PERL_JSON_BACKEND=Cpanel::JSON::XS,JSON::XS,JSON::PP

You may also want to set this variable at the top of your test files, in order
not to be bothered with incompatibilities between backends (you need to wrap
this in "BEGIN", and set before actually "use"-ing JSON
module, as it decides its backend as soon as it's loaded):

With this option, this module loads its pure perl backend along with its XS
backend (if available), and lets the XS backend to watch if you set a flag
only JSON::PP supports. When you do, the internal JSON::XS object is
replaced with a newly created JSON::PP object with the setting copied from
the XS object, so that you can use JSON::PP flags (and its slower
"decode"/"encode" methods) from then on. In other
words, this is not something that allows you to hook JSON::XS to change
its behavior while keeping its speed. JSON::XS and JSON::PP objects are
quite different (JSON::XS object is a blessed scalar reference, while
JSON::PP object is a blessed hash reference), and can't share their
internals.

To avoid needless overhead (by copying settings), you are advised not to use
this option and just to use JSON::PP explicitly when you need JSON::PP
features.

JSON::XS-compatible backend modules don't encode blessed objects by default
(except for their boolean values, which are typically blessed
JSON::PP::Boolean objects). If you need to encode a data structure that
may contain objects, you usually need to look into the structure and
replace objects with alternative non-blessed values, or enable
"convert_blessed" and provide a "TO_JSON" method for
each object's (base) class that may be found in the structure, in order to
let the methods replace the objects with whatever scalar values the
methods return.

If you need to serialise data structures that may contain arbitrary objects,
it's probably better to use other serialisers (such as Sereal or Storable
for example), but if you do want to use this module for that purpose,
"-convert_blessed_universally" option may help, which tweaks
"encode" method of the backend to install
"UNIVERSAL::TO_JSON" method (locally) before encoding, so that
all the objects that don't have their own "TO_JSON" method can
fall back on the method in the "UNIVERSAL" namespace. Note that
you still need to enable "convert_blessed" flag to actually
encode objects in a data structure, and "UNIVERSAL::TO_JSON"
method installed by this option only converts blessed hash/array
references into their unblessed clone (including private keys/values that
are not supposed to be exposed). Other blessed references will be
converted into null.

This feature is experimental and may be removed in the future.

-no_export

When you don't want to import functional interfaces from a module, you
usually supply "()" to its "use" statement.

use JSON (); # no functional interfaces

If you don't want to import functional interfaces, but you also want to use
any of the above options, add "-no_export" to the option list.

# no functional interfaces, while JSON::PP support is enabled.
use JSON -support_by_pp, -no_export;

This section is taken from JSON::XS. "encode_json" and
"decode_json" are exported by default.

This module also exports "to_json" and "from_json" for
backward compatibility. These are slower, and may expect/generate different
stuff from what "encode_json" and "decode_json" do,
depending on their options. It's better just to use Object-Oriented interfaces
than using these two functions.

If $enable is true (or missing), then the "encode" method will not
generate characters outside the code range 0..127 (which is ASCII). Any
Unicode characters outside that range will be escaped using either a single
\uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence, as per
RFC4627. The resulting encoded JSON text can be treated as a native Unicode
string, an ascii-encoded, latin1-encoded or UTF-8 encoded string, or any other
superset of ASCII.

If $enable is false, then the "encode" method will not escape Unicode
characters unless required by the JSON syntax or other flags. This results in
a faster and more compact format.

See also the section ENCODING/CODESET FLAG NOTES later in this document.

The main use for this flag is to produce JSON texts that can be transmitted over
a 7-bit channel, as the encoded JSON texts will not contain any 8 bit
characters.

If $enable is true (or missing), then the "encode" method will encode
the resulting JSON text as latin1 (or iso-8859-1), escaping any characters
outside the code range 0..255. The resulting string can be treated as a
latin1-encoded JSON text or a native Unicode string. The "decode"
method will not be affected in any way by this flag, as "decode" by
default expects Unicode, which is a strict superset of latin1.

If $enable is false, then the "encode" method will not escape Unicode
characters unless required by the JSON syntax or other flags.

See also the section ENCODING/CODESET FLAG NOTES later in this document.

The main use for this flag is efficiently encoding binary data as JSON text, as
most octets will not be escaped, resulting in a smaller encoded size. The
disadvantage is that the resulting JSON text is encoded in latin1 (and must
correctly be treated as such when storing and transferring), a rare encoding
for JSON. It is therefore most useful when you want to store data structures
known to contain binary data efficiently in files or databases, not when
talking to other JSON encoders/decoders.

If $enable is true (or missing), then the "encode" method will encode
the JSON result into UTF-8, as required by many protocols, while the
"decode" method expects to be handled an UTF-8-encoded string.
Please note that UTF-8-encoded strings do not contain any characters outside
the range 0..255, they are thus useful for bytewise/binary I/O. In future
versions, enabling this option might enable autodetection of the UTF-16 and
UTF-32 encoding families, as described in RFC4627.

If $enable is false, then the "encode" method will return the JSON
string as a (non-encoded) Unicode string, while "decode" expects
thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16)
needs to be done yourself, e.g. using the Encode module.

See also the section ENCODING/CODESET FLAG NOTES later in this document.

If $enable is true (or missing), then the "encode" method will add an
extra optional space after the ":" separating keys from values in
JSON objects and extra whitespace after the "," separating key-value
pairs and array members.

If $enable is false, then the "encode" method will not add any extra
space at those places.

If $enable is true (or missing), then "decode" will accept some
extensions to normal JSON syntax (see below). "encode" will not be
affected in anyway. Be aware that this option makes you accept invalidJSON texts as if they were valid!. I suggest only to use this option to
parse application-specific files written by humans (configuration files,
resource files etc.)

If $enable is false (the default), then "decode" will only accept
valid JSON texts.

Currently accepted extensions are:

•

list items can have an end-comma

JSON separates array elements and key-value pairs with commas. This
can be annoying if you write JSON texts manually and want to be able to
quickly append elements, so this extension accepts comma at the end of
such items not just between them:

Whenever JSON allows whitespace, shell-style comments are additionally
allowed. They are terminated by the first carriage-return or line-feed
character, after which more white-space and comments are allowed.

If $enable is true (or missing), then the "encode" method will output
JSON objects by sorting their keys. This is adding a comparatively high
overhead.

If $enable is false, then the "encode" method will output key-value
pairs in the order Perl stores them (which will likely change between runs of
the same script, and can change even within the same run from 5.18 onwards).

This option is useful if you want the same data structure to be encoded as the
same JSON text (given the same overall settings). If it is disabled, the same
hash might be encoded differently even if contains the same data, as key-value
pairs have no inherent ordering in Perl.

If $enable is true (or missing), then the "encode" method can convert
a non-reference into its corresponding string, number or null JSON value,
which is an extension to RFC4627. Likewise, "decode" will accept
those JSON values instead of croaking.

If $enable is false, then the "encode" method will croak if it isn't
passed an arrayref or hashref, as JSON texts must either be an object or
array. Likewise, "decode" will croak if given something that is not
a JSON object or array.

If $enable is true (or missing), then "encode" will not throw
an exception when it encounters values it cannot represent in JSON (for
example, filehandles) but instead will encode a JSON "null" value.
Note that blessed objects are not included here and are handled separately by
c<allow_nonref>.

If $enable is false (the default), then "encode" will throw an
exception when it encounters anything it cannot encode as JSON.

This option does not affect "decode" in any way, and it is recommended
to leave it off unless you know your communications partner.

If $enable is true (or missing), then the "encode" method will not
barf when it encounters a blessed reference that it cannot convert otherwise.
Instead, a JSON "null" value is encoded instead of the object.

If $enable is false (the default), then "encode" will throw an
exception when it encounters a blessed object that it cannot convert
otherwise.

If $enable is true (or missing), then "encode", upon encountering a
blessed object, will check for the availability of the "TO_JSON"
method on the object's class. If found, it will be called in scalar context
and the resulting scalar will be encoded instead of the object.

The "TO_JSON" method may safely call die if it wants. If
"TO_JSON" returns other blessed objects, those will be handled in
the same way. "TO_JSON" must take care of not causing an endless
recursion cycle (== crash) in this case. The name of "TO_JSON" was
chosen because other methods called by the Perl core (== not by the user of
the object) are usually in upper case letters and to avoid collisions with any
"to_json" function or method.

If $enable is false (the default), then "encode" will not consider
this type of conversion.

When $coderef is specified, it will be called from "decode" each time
it decodes a JSON object. The only argument is a reference to the
newly-created hash. If the code references returns a single scalar (which need
not be a reference), this value (i.e. a copy of that scalar to avoid aliasing)
is inserted into the deserialised data structure. If it returns an empty list
(NOTE: not "undef", which is a valid scalar), the original
deserialised hash will be inserted. This setting can slow down decoding
considerably.

When $coderef is omitted or undefined, any existing callback will be removed and
"decode" will not change the deserialised hash in any way.

Works remotely similar to "filter_json_object", but is only called for
JSON objects having a single key named $key.

This $coderef is called before the one specified via
"filter_json_object", if any. It gets passed the single value in the
JSON object. If it returns a single value, it will be inserted into the data
structure. If it returns nothing (not even "undef" but the empty
list), the callback from "filter_json_object" will be called next,
as if no single-key callback were specified.

If $coderef is omitted or undefined, the corresponding callback will be
disabled. There can only ever be one callback for a given key.

As this callback gets called less often then the "filter_json_object"
one, decoding speed will not usually suffer as much. Therefore, single-key
objects make excellent targets to serialise Perl objects into, especially as
single-key JSON objects are as close to the type-tagged value concept as JSON
gets (it's basically an ID/VALUE tuple). Of course, JSON does not support this
in any way, so you need to make sure your data never looks like a serialised
Perl hash.

Typical names for the single object key are "__class_whatever__", or
"$__dollars_are_rarely_used__$" or
"}ugly_brace_placement", or even things like
"__class_md5sum(classname)__", to reduce the risk of clashing with
real hashes.

Sets the maximum nesting level (default 512) accepted while encoding or
decoding. If a higher nesting level is detected in JSON text or a Perl data
structure, then the encoder and decoder will stop and croak at that point.

Nesting level is defined by number of hash- or arrayrefs that the encoder needs
to traverse to reach a given point or the number of "{" or
"[" characters without their matching closing parenthesis crossed to
reach a given character in a string.

Setting the maximum depth to one disallows any nesting, so that ensures that the
object is only a single hash/object or array.

If no argument is given, the highest possible setting will be used, which is
rarely useful.

Set the maximum length a JSON text may have (in bytes) where decoding is being
attempted. The default is 0, meaning no limit. When "decode" is
called on a string that is longer then this many bytes, it will not attempt to
decode the string but throw an exception. This setting has no effect on
"encode" (yet).

If no argument is given, the limit check will be deactivated (same as when 0 is
specified).

This works like the "decode" method, but instead of raising an
exception when there is trailing garbage after the first JSON object, it will
silently stop parsing there and return the number of characters consumed so
far.

This is useful if your JSON texts are not delimited by an outer protocol and you
need to know where the JSON text ends.

Since 2.92, "backend" method returns an abstract backend module used
currently, which should be JSON::Backend::XS (which inherits JSON::XS or
Cpanel::JSON::XS), or JSON::Backend::PP (which inherits JSON::PP), not to
monkey-patch the actual backend module globally.

If you need to know what is used actually, use "isa", instead of
string comparison.

In some cases, there is the need for incremental parsing of JSON texts. While
this module always has to keep both JSON text and resulting Perl data
structure in memory at one time, it does allow you to parse a JSON stream
incrementally. It does so by accumulating text until it has a full JSON
object, which it then can decode. This process is similar to using
"decode_prefix" to see if a full JSON object is available, but is
much more efficient (and can be implemented with a minimum of method calls).

This module will only attempt to parse the JSON text once it is sure it has
enough text to get a decisive result, using a very simple but truly
incremental parser. This means that it sometimes won't stop as early as the
full parser, for example, it doesn't detect mismatched parentheses. The only
thing it guarantees is that it starts decoding as soon as a syntactically
valid JSON text has been seen. This means you need to set resource limits
(e.g. "max_size") to ensure the parser will stop parsing in the
presence if syntax errors.

This is the central parsing function. It can both append new text and extract
objects from the stream accumulated so far (both of these functions are
optional).

If $string is given, then this string is appended to the already existing JSON
fragment stored in the $json object.

After that, if the function is called in void context, it will simply return
without doing anything further. This can be used to add more text in as many
chunks as you want.

If the method is called in scalar context, then it will try to extract exactly
one JSON object. If that is successful, it will return this object,
otherwise it will return "undef". If there is a parse error, this
method will croak just as "decode" would do (one can then use
"incr_skip" to skip the erroneous part). This is the most common way
of using the method.

And finally, in list context, it will try to extract as many objects from the
stream as it can find and return them, or the empty list otherwise. For this
to work, there must be no separators (other than whitespace) between the JSON
objects or arrays, instead they must be concatenated back-to-back. If an error
occurs, an exception will be raised as in the scalar context case. Note that
in this case, any previously-parsed JSON texts will be lost.

Example: Parse some JSON arrays/objects in a given string and return them.

This method returns the currently stored JSON fragment as an lvalue, that is,
you can manipulate it. This only works when a preceding call to
"incr_parse" in scalar context successfully returned an
object. Under all other circumstances you must not call this function (I mean
it. although in simple tests it might actually work, it will fail under
real world conditions). As a special exception, you can also call this method
before having parsed anything.

That means you can only use this function to look at or manipulate text before
or after complete JSON objects, not while the parser is in the middle of
parsing a JSON object.

This function is useful in two cases: a) finding the trailing text after a JSON
object or b) parsing multiple JSON objects separated by non-JSON text (such as
commas).

This will reset the state of the incremental parser and will remove the parsed
text from the input buffer so far. This is useful after "incr_parse"
died, in which case the input buffer and incremental parser state is left
unchanged, to skip the text parsed so far and to reset the parse state.

The difference to "incr_reset" is that only text until the parse error
occurred is removed.

This section describes how the backend modules map Perl values to JSON values
and vice versa. These mappings are designed to "do the right thing"
in most circumstances automatically, preserving round-tripping characteristics
(what you put in comes out as something equivalent).

For the more enlightened: note that in the following descriptions, lowercase
perl refers to the Perl interpreter, while uppercase Perl refers
to the abstract Perl language itself.

A JSON object becomes a reference to a hash in Perl. No ordering of object
keys is preserved (JSON does not preserver object key ordering
itself).

array

A JSON array becomes a reference to an array in Perl.

string

A JSON string becomes a string scalar in Perl - Unicode codepoints in JSON
are represented by the same codepoints in the Perl string, so no manual
decoding is necessary.

number

A JSON number becomes either an integer, numeric (floating point) or
string scalar in perl, depending on its range and any fractional parts. On
the Perl level, there is no difference between those as Perl handles all
the conversion details, but an integer may take slightly less memory and
might represent more values exactly than floating point numbers.

If the number consists of digits only, this module will try to represent it
as an integer value. If that fails, it will try to represent it as a
numeric (floating point) value if that is possible without loss of
precision. Otherwise it will preserve the number as a string value (in
which case you lose roundtripping ability, as the JSON number will be
re-encoded to a JSON string).

Numbers containing a fractional or exponential part will always be
represented as numeric (floating point) values, possibly at a loss of
precision (in which case you might lose perfect roundtripping ability, but
the JSON number will still be re-encoded as a JSON number).

Note that precision is not accuracy - binary floating point values cannot
represent most decimal fractions exactly, and when converting from and to
floating point, this module only guarantees precision up to but not
including the least significant bit.

true, false

These JSON atoms become "JSON::true" and
"JSON::false", respectively. They are overloaded to act almost
exactly like the numbers 1 and 0. You can check whether a scalar is a JSON
boolean by using the "JSON::is_bool" function.

null

A JSON null atom becomes "undef" in Perl.

shell-style comments ("# text")

As a nonstandard extension to the JSON syntax that is enabled by the
"relaxed" setting, shell-style comments are allowed. They can
start anywhere outside strings and go till the end of the line.

The mapping from Perl to JSON is slightly more difficult, as Perl is a truly
typeless language, so we can only guess which JSON type is meant by a Perl
value.

hash references

Perl hash references become JSON objects. As there is no inherent ordering
in hash keys (or JSON objects), they will usually be encoded in a
pseudo-random order. This module can optionally sort the hash keys
(determined by the canonical flag), so the same data structure will
serialise to the same JSON text (given same settings and version of the
same backend), but this incurs a runtime overhead and is only rarely
useful, e.g. when you want to compare some JSON text against another for
equality.

array references

Perl array references become JSON arrays.

other references

Other unblessed references are generally not allowed and will cause an
exception to be thrown, except for references to the integers 0 and 1,
which get turned into "false" and "true" atoms in
JSON. You can also use "JSON::false" and "JSON::true"
to improve readability.

encode_json [\0,JSON::true] # yields [false,true]

JSON::true, JSON::false, JSON::null

These special values become JSON true and JSON false values, respectively.
You can also use "\1" and "\0" directly if you
want.

blessed objects

Blessed objects are not directly representable in JSON, but
"JSON::XS" allows various ways of handling objects. See
"OBJECT SERIALISATION", below, for details.

simple scalars

Simple Perl scalars (any scalar that is not a reference) are the most
difficult objects to encode: this module will encode undefined scalars as
JSON "null" values, scalars that have last been used in a string
context before encoding as JSON strings, and anything else as number
value:

my $x = "3"; # some variable containing a string
$x += 0; # numify it, ensuring it will be dumped as a number
$x *= 1; # same thing, the choice is yours.

You can not currently force the type in other, less obscure, ways. Tell me
if you need this capability (but don't forget to explain why it's needed
:).

Note that numerical precision has the same meaning as under Perl (so binary
to decimal conversion follows the same rules as in Perl, which can differ
to other languages). Also, your perl interpreter might expose extensions
to the floating point numbers of your platform, such as infinities or
NaN's - these cannot be represented in JSON, and it is an error to pass
those in.

The interested reader might have seen a number of flags that signify encodings
or codesets - "utf8", "latin1" and "ascii".
There seems to be some confusion on what these do, so here is a short
comparison:

"utf8" controls whether the JSON text created by "encode"
(and expected by "decode") is UTF-8 encoded or not, while
"latin1" and "ascii" only control whether
"encode" escapes character values outside their respective codeset
range. Neither of these flags conflict with each other, although some
combinations make less sense than others.

Care has been taken to make all flags symmetrical with respect to
"encode" and "decode", that is, texts encoded with any
combination of these flag values will be correctly decoded when the same flags
are used - in general, if you use different flag settings while encoding vs.
when decoding you likely have a bug somewhere.

Below comes a verbose discussion of these flags. Note that a "codeset"
is simply an abstract set of character-codepoint pairs, while an encoding
takes those codepoint numbers and encodes them, in our case into
octets. Unicode is (among other things) a codeset, UTF-8 is an encoding, and
ISO-8859-1 (= latin 1) and ASCII are both codesets and encodings at the
same time, which can be confusing.

"utf8" flag disabled

When "utf8" is disabled (the default), then
"encode"/"decode" generate and expect Unicode strings,
that is, characters with high ordinal Unicode values (> 255) will be
encoded as such characters, and likewise such characters are decoded
as-is, no changes to them will be done, except
"(re-)interpreting" them as Unicode codepoints or Unicode
characters, respectively (to Perl, these are the same thing in strings
unless you do funny/weird/dumb stuff).

This is useful when you want to do the encoding yourself (e.g. when you want
to have UTF-16 encoded JSON texts) or when some other layer does the
encoding for you (for example, when printing to a terminal using a
filehandle that transparently encodes to UTF-8 you certainly do NOT want
to UTF-8 encode your data first and have Perl encode it another
time).

"utf8" flag enabled

If the "utf8"-flag is enabled,
"encode"/"decode" will encode all characters using the
corresponding UTF-8 multi-byte sequence, and will expect your input
strings to be encoded as UTF-8, that is, no "character" of the
input string must have any value > 255, as UTF-8 does not allow that.

The "utf8" flag therefore switches between two modes: disabled
means you will get a Unicode string in Perl, enabled means you get an
UTF-8 encoded octet/binary string in Perl.

"latin1" or "ascii" flags enabled

With "latin1" (or "ascii") enabled, "encode"
will escape characters with ordinal values > 255 (> 127 with
"ascii") and encode the remaining characters as specified by the
"utf8" flag.

If "utf8" is disabled, then the result is also correctly encoded
in those character sets (as both are proper subsets of Unicode, meaning
that a Unicode string with all character values < 256 is the same thing
as a ISO-8859-1 string, and a Unicode string with all character values
< 128 is the same thing as an ASCII string in Perl).

If "utf8" is enabled, you still get a correct UTF-8-encoded
string, regardless of these flags, just some more characters will be
escaped using "\uXXXX" then before.

Note that ISO-8859-1- encoded strings are not compatible with UTF-8
encoding, while ASCII-encoded strings are. That is because the ISO-8859-1
encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 codeset
being a subset of Unicode), while ASCII is.

Surprisingly, "decode" will ignore these flags and so treat all
input values as governed by the "utf8" flag. If it is disabled,
this allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
strict subsets of Unicode. If it is enabled, you can correctly decode
UTF-8 encoded strings.

So neither "latin1" nor "ascii" are incompatible with
the "utf8" flag - they only govern when the JSON output engine
escapes a character or not.

The main use for "latin1" is to relatively efficiently store
binary data as JSON, at the expense of breaking compatibility with most
JSON decoders.

The main use for "ascii" is to force the output to not contain
characters with values > 127, which means you can interpret the
resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any
character set and 8-bit-encoding, and still get the same data structure
back. This is useful when your channel for JSON transfer is not 8-bit
clean or the encoding might be mangled in between (e.g. in mail), and
works because ASCII is a proper subset of most 8-bit and multibyte
encodings in use in the world.

Since version 2.90, stringification (and string comparison) for
"JSON::true" and "JSON::false" has not been overloaded. It
shouldn't matter as long as you treat them as boolean values, but a code that
expects they are stringified as "true" or "false" doesn't
work as you have expected any more.

if (JSON::true eq 'true') { # now fails
print "The result is $JSON::true now."; # => The result is 1 now.

And now these boolean values don't inherit JSON::Boolean, either. When you need
to test a value is a JSON boolean value or not, use "JSON::is_bool"
function, instead of testing the value inherits a particular boolean class or
not.